Crystals such as lithium niobate or lithium tantalate are used in components for laser optics, for example, in frequency doublers or frequency mixers. However, in general there exists the problem that under intense irradiation with laser light, the light beam passing through the crystal is distorted and thus loses its quality. The efficiency of the frequency conversion drops. The reason for the loss of beam quality lies in the fact that the laser light excites charge carriers that are then redistributed and captured at other locations. This causes the buildup of space-charge fields that produce local modulations of the index of refraction, at which the light beam fans out. In addition, the charge carrier transfer can cause a change in absorption if the capturers of the charges have a different photon absorption cross-section. As a result of the Kramers-Kronig relationships for the complex index of refraction, the index of refraction is also affected by these absorption changes. The inhomogeneous changes in the index of refraction and the light-induced absorption changes are referred to as “optical damage.”
Hereinafter, when reference is made to electrons, it is intended to apply quite generally to electronic charges. This can include both electron and hole transport.
The charge redistribution is largely caused by contaminants with ions that can take on multiple valence states within the crystal and are thus both electron donors and electron traps. For example, iron, which occurs in lithium niobate and lithium tantalate in the valence states Fe2+ and Fe3+, is such a contaminant. Here, the charges are excited from Fe2+ lattice defects and are transported through the conduction band to places where Fe3+ ions trap these electrons. Intrinsic crystal defects occurring in multiple valence states can promote this process or can make it possible on their own. In lithium niobate, for example, there is what is known as the “antisite defect,” where Nb5+/4+ sits at a Li site. In addition, the charges can be trapped in centers where they lead to an increase in absorption. All of these effects are also considered optical damage.
The cause of optical damage is thus, most generally, that defects where electrons can be excited by light are present in the crystals. On the other hand, this means that the higher the purity of the crystal, and the fewer opportunities for charge excitation it offers, the smaller the optical damage will be.
Now, the avoidance of undesired contaminants as early as the growing of the crystals, to the extent that this can be done, is known. However, it has proven to be very difficult to grow crystals with a doping of less than 10 ppm of unwanted ions. It is also known to purify the crystal of excitable electrons after the fact by thermal or thermoelectric oxidation, in that the electron-donating ions are oxidized, which is to say that the Fe2+ ions are converted to the Fe3+ state. In terms of charge, the excess electrons that are liberated are compensated by the inclusion or exclusion of ions. These methods are known as annealing, wherein their purifying effect is made possible by heating of the crystal to high temperatures. External electrical voltages applied to the crystal can assist the process. Here, the concentration ratio of filled to empty lattice defects serves as a measure of the oxidation. At low concentration ratios, only correspondingly little charge can be transported, since the electron donors are lacking. Accordingly, no space charge field is developed. Moreover, fewer electrons can be transported to centers that lead to stronger absorption. Optical damage is thus reduced. However, the optical damage may possibly appear again above a certain “threshold intensity,” because new possibilities for excitation are created, for example, by multiple-photon excitation, and other centers take part in charge transport.
To reduce optical damage, the crystals can also be doped with magnesium, for example; however, this doping degrades other properties. Thus, for example, it becomes significantly more difficult to produce “periodically polarized” crystals in which the spontaneous polarization is periodically inverted. However, this periodic polarization is a prerequisite for numerous nonlinear optical applications such as frequency conversion with the aid of “quasi-phase matching.”
Even though the prior art methods have a significant purifying effect, the purity of the treated crystals is frequently inadequate for applications with high-intensity laser beams. For this reason, it has also scarcely been possible to date to employ nonlinear optical or electro-optical components such as, for example, frequency doublers, frequency mixers, optical parametric oscillators (OPOs), or electro-optic modulators in conjunction with high-intensity laser beams. The optical damage to which these laser beams expose the components has to date impaired their functionality to an unacceptable degree, or else has made the production of the components significantly more difficult or costly as a result of the aforementioned co-doping with magnesium or other elements.